1. Field of the Invention
The invention relates to methods for studying and utilizing flow waveforms in the peripheral vasculature. In particular, the invention relates to a method for assessing blood volume by analyzing photoelectric plethysmographic waveforms of the pulse oximeter.
2. Description of the Prior Art
There is growing evidence that invasive monitors of volume status, such as the pulmonary artery catheter, may be a source of unacceptably frequent complications. Dalen J & Bone R, Is it time to pull the pulmonary artery catheter?, JAMA 1996; 276:916-14; Connors A, Speroff T & Dawson N, The effectiveness of right heart catheterization in the initial care of critically ill patients, JAMA 1996; 276:889-97. The potential loss of this important monitor from routine perioperative care necessitates the search for another means of monitoring a patient's blood volume status.
It has been known for quite some time that ventilation, and especially positive pressure ventilation, can have a significant impact on the cardiovascular system. Cournand A, Motley H, Werko L & Richards D, Physiological studies of the effect of intermittent positive pressure breathing an cardiac output in man, Am J Physiol 1948; 152:162-73; Morgan B, Crawford W & Guntheroth W, The hemodynamic effects of changes in blood volume during intermittent positive-pressure ventilation, Anesthesiology 1969; 30:297-305. The first formal studies of the effect of ventilator induced changes on the arterial pressure were done in the early 1980's. Coyle J, Teplick R. Long M & Davison J, Respiratory variations in systemic arterial pressure as an indicator of volume status, Anesthesiology 1983; 59:A53; Jardin F, Farcot J, Gueret P et al., Cyclic changes in arterial pulse during respiratory support, Circulation 1983; 68:266-74. This was soon followed by the intensive investigations of Azriel Perel. He coined the term “systolic pressure variation” to describe this phenomenon. Along with various co-investigators, his research has encompassed over twenty articles and abstracts on the topic. From this significant body of work, based on both animal and human data, a number of conclusions have been drawn.
The degree of systolic pressure variation is a sensitive indicator of hypo Perel A, Pizov R & Cotev S, Systolic blood variation is a sensitive indicator of hypovelemia in ventilated dogs subjected to graded hemorrhage, Anesthesiology 1987; 67:498-502. This variation is significantly better than heart rate, central venous pressure and mean systemic blood pressure in predicting the degree of hemorrhage which has occurred. Perel A, Pizov R & Cotev S, Systolic blood pressure variation is a sensitive indicator of hypovelemia in ventilated dogs subjected to graded hemorrhage Anesthesiology 1987; 67:498-502; Pizov R. Ya'ari Y & Perel A, Systolic pressure variation is greater during hemorrhage than during sodium nitroprusside-induced hypotension in ventilated dogs, Anesthesia & Analgesia 1988; 67:170-4. Chest wall compliance and tidal volume can influence systolic pressure variation. Szold A, Pizov R, Segal E & Perel A, The effect of tidal volume and intravascular volume state on systolic pressure variation in ventilated dogs, Intensive Care Medicine 1989; 15:368-71. Changes in systolic pressure variation correspond closely to changes in cardiac output. Ornstein E, Eidelman L, Drenger B et al., Systolic pressure variation predicts the response to acute blood loss, Journal of Clinical Anesthesia 1998; 10:137-40; Pizov R, Segal E, Kaplan L et al., The use of systolic pressure variation in hemodynamic monitoring during deliberate hypotension in spine surgery, Journal of Clinical Anesthesia 1990; 2:96-100.
Systolic pressure variation can be divided into two distinct components; Δup, which reflects an inspiratory augmentation of the cardiac output, and Δdown, which reflects a reduction in cardiac output due to a decrease in venous return. Perel A, Cardiovascular assessment by pressure waveform analysis, ASA Annual Refresher Course Lecture 1991:264. The unique value in systolic pressure variation lies in its ability to reflect the volume responsiveness of the left ventricle. Perel A, Cardiovascular assessment by pressure waveform analysis, ASA Annual Refresher Course Lecture 1991:264. In recent years, with the increased availability of the pulse oximeter waveform, similar observations have been made with this monitoring system. Partridge B L, Use of pulse oximetry as a noninvasive indicator of intravascular volume status, Journal of Clinical Monitoring 1987; 3:263-8; Lherm T, Chevalier T, Troche G et al., Correlation between plethysmography curve variation (dpleth) and pulmonary capillary wedge pressure (pcup) in mechanically ventilated patients, British Journal of Anesthesia 1995; Suppl. 1:41; Shamir M, Eidelman L A et al., Pulse oximetry plethysmographic waveform during changes in blood volume, British Journal Of Anaesthesia 82(2): 178-81 (1999). To date though, there has been remarkably little work done to document or quantify this phenomenon. Limitations of the aforementioned include:
1) Lack of a method of continuous measurement of the phenomenon.
2) Reliance on positive pressure and mechanical ventilation; and the requirement of ventilator maneuvers such as periods of apnea.
3) Lack of recognition of the venous contribution to the plethysmographic signal.
4) The lack of algorithms resistant to artifacts.
The pulse oximeter has rapidly become one of the most commonly used patient monitoring systems both in and out of the operating room. This popularity is undoubtedly due to the pulse oximeter's ability to monitor both arterial oxygen saturation as well as basic cardiac function (i.e. heart rhythm) non-invasively. In addition, it is remarkably easy to use and comfortable for the patient. The present invention attempts to expand upon the known usefulness of the pulse oximeter.
Pulse oximetry is a simple non-invasive method of monitoring the percentage of hemoglobin (Hb) which is saturated with oxygen. The pulse oximeter consists of a probe attached to the patient's finger or ear lobe, which is linked to a computerized unit. The unit displays the percentage of Hb saturated with oxygen together with an audible signal for each pulse beat, a calculated heart rate and in some models, a graphical display of the blood flow past the probe. Audible alms that can be programmed by the user are provided. A source of light originates from the probe at two wavelengths (e.g., 650 nm and 805 nm). The light is partly absorbed by hemoglobin, by amounts which differ depending on whether it is saturated with oxygen. By calculating the absorption at the two wavelengths the processor can compute the proportion of hemoglobin which is oxygenated. The oximeter is dependent on a pulsatile flow and produces a graph of the quality of flow. Where flow is compromised (e.g. by hypovolemia or vasoconstriction), the pulse oximeter may be unable to function. The computer within the oximeter is capable of distinguishing pulsatile flow from other more static signals (such as tissue or venous signals) to display only the arterial flow. Fearnley S J, Pulse Oximetry, Practical Procedures, Issue 5 (1995) Article 2.
In the process of determining oxygen saturation, the pulse oximeter functions as a photoelectric plethysmograph. In this role, it non-invasively measures minute changes in the blood volume of a vascular bed (e.g., finger, ear or forehead). The photoelectric plethysmograph is not a new invention. Hertzman A B, The Blood Supply of Various Skin Areas as Estimated By the Photoelectric Plethysmograph, Am. J. Physiol. 1938; 124:328-40. While the plethysmograph has been examined previously as a potential anesthesia monitoring device, remarkably little research has been done on this ubiquitous signal Dorlas J C & Nijboer J A, Photo-electric plethysmography as a monitoring device in anaesthesia. Application and interpretation, British Journal Of Anaesthesia 1985; 57:524-30.
It is important to understand that the typical pulse oximeter waveform presented to the clinician is a highly filtered and processed signal. It is normal practice for equipment manufactures to use both auto-centering and auto-gain routines on the displayed waveforms so as to minimize variations in the displayed signal. Despite this fact, the pulse oximeter waveform is still rich in information regarding the physiology of the patient. It contains a complex mixture of the influences of arterial, venous, autonomic and respiratory systems on the peripheral circulation. Key to the successful interpretation of this waveform is the ability to separate it into fundamental components.
Harmonic analysis (Fourier analysis) is one method of studying waveforms. It allows for the extraction of underlying signals that contribute to a complex waveform. A similar method has been used previously, with the pulse oximeter and photoelectric plethysmograph to improve the accuracy of the oxygen saturation measurement and to monitor tissue perfusion. Rusch T L, Sankar R & Scharf J E, Signal processing methods for pulse oximetry, Computers In Biology And Medicine 1996; 26:143-59; Stack B Jr, Futran N D, Shohet M N & Scharf J E, Spectral analysis of photoplethysmograms from radial forearm free flaps, Laryngoscope 1998; 108:1329-33.
In addition, it is previously know that photoelectric plethysmograph can be used to non-invasively measure minute changes in light absorption of living tissue. Hertzman, A B, The Blood Supply of Various Skin Areas as Estimated By the Photoelectric Plethysmograph, Am. J. Physiol. 124: 328-340 (1938). Rhythmic fluctuations in this signal are normally attributed to the cardiac pulse bringing more blood into the region being analyzed (i.e., finger, ear or forehead). This fluctuation of the plethysmographic signal is commonly referred to as the AC (arterial) component. The strength of the AC component can be modulated by a variety of factors. These factors would include stroke volume and vascular tone.
In addition to the cardiac pulse, there is a nonpulsatile (or weakly pulsatile) component of the plethysmography signal commonly referred to as the DC component. The DC component is the product the light absorption by nonpulsatile tissue. This would include fat, bone, muscle and venous blood.
Fluctuations in the photoelectric plethysmograph due to respiration/ventilation can also be detected. Johansson A & Oberg P A, Estimation of respiratory volumes from the photoplethysmographic sit. Part I: Experimental results, Medical And Biological Engineering And Computing 37(1): 42-7 (1999). These fluctuations have been used in the past in an attempt to estimate the degree of relative blood volume of patients undergoing surgery. Partridge B L, Use of pulse oximetry as a noninvasive indicator of intravascular volume status, Journal of Clinical Monitoring 3(4): 263-8 (1987); Shamir M, Eidelman L A et al., Pulse oximetry plethysmographic waveform during changes in blood volume, British Journal Of Anaesthesia 82(2): 178-81 (1999). Up until the development of the present invention, there has been a lack of recognition that respiratory signal modulates both the AC and DC components of the photo-plethysmograph.
With the foregoing in mind, it is apparent that a need exists for an improved method for efficiently and non-invasively monitoring the blood volume of a subject. The present invention attempts to achieve this goal by expanding upon the functionality of the pulse oximeter to provide a reliable, convenient and non-invasive mechanism for monitoring blood loss. In addition, the present invention provides new methods for extracting and utilizing information contained in a pulse oximeter waveform and other means of monitoring flow or pressure waveforms in the peripheral vasculature.